1. Field of the Invention
The present invention relates to a data distribution system and devices used in the system, and more particularly to a system in which multimedia data is broadcast (transmitted) from a transmission device by broadcast or communication and the broadcast (transmitted) multimedia data is received at a receiving device and displayed on a screen, and to the devices used in this system.
2. Description of the Background Art
Various terminal devices which obtain and display distributed multimedia data have been conventionally developed. Among such devices, terminal devices used on the Internet obtain the multimedia data as follows. First, according to an instruction from a user, the terminal device directly accesses through the Internet a WWW (World Wide Web) server in which multimedia data are previously stored. The terminal device then retrieves necessary multimedia data from the WWW server.
In this way, the main current of the system for obtaining multimedia data has been the method of so-called “pull” type, where users freely retrieve target data typically through the Internet as stated above.
On the other hand, the demand for multimedia data distribution through broadcasting is currently growing, which belongs to “push” type where data is pushed out toward users. The data distribution through DAB (Digital Audio Broadcasting) which was developed in the EUREKA-147 project in Europe is an example of the push type. In such multimedia data distribution through broadcasting, a user can obtain necessary multimedia data by indicating to a receiving terminal a broadcast channel to receive the data. That is to say, necessary multimedia data is automatically received through the specified channel and displayed.
The user of the receiving terminal can thus easily refer to the multimedia data just like through conventional broadcast on TV or radio. In this way, the multimedia data distribution through broadcasting has the advantage of saving the need for executing the process of searching for and taking out necessary data.
However, the above-explained multimedia data distribution through broadcasting differs from the real time data distribution from WWW servers on the Internet (distribution where a data distribution response to a data reference request can be made in real time). A broadcasting station in the broadcast multimedia data distribution therefore cannot process reference requests from users in real time. It is hence difficult for the users to control the contents of the broadcast multimedia data in the broadcast multimedia data distribution.
That is to say, when users demand to refer to arbitrary multimedia data, it is difficult for the broadcasting station in the data broadcasting system to satisfy every reference request in real time. Accordingly the users must wait until multimedia data they want to refer to are broadcast according to a predetermined broadcast program to obtain the necessary multimedia data.
Now, if data can be stored in the receiving terminal, the user can locally refer to the multimedia data broadcast in the past (multimedia data stored in the receiving terminal). However, although it depends on the circuit scale of the terminal and the cost, the receiving terminal is usually capable of storing only a small part of the data stored in the data broadcast equipment in the broadcasting station. Thus, although the multimedia data can be locally referred to, the user is actually allowed just to fragmentarily and time-sequentially receive and refer to only part of the multimedia data existing in the data broadcasting equipment.
For example, when the multimedia data to be referred to is in the HTML (Hyper Text Markup Language) format having a link structure, the users cannot conveniently refer to the multimedia data until they receive other multimedia data linked to the multimedia data (referred to as linked data hereinafter).
Moreover, when the linked data is absent in the data broadcasting equipment, e.g., when the linked data resides in an external WWW server, the user cannot even known the outlines of the data, not to mention referring to the linked data.
As stated above, in the multimedia data distribution through broadcasting, users cannot know what multimedia data will be broadcast and have to wait passively. The multimedia data distribution through broadcasting thus has the problem that the users cannot grasp the entire structure of the broadcast multimedia data and therefore have difficulty in judging which multimedia data are necessary. That it to say, the users cannot instantaneously decide whether data linked to the multimedia data currently referred to is necessary, and therefore they cannot decide whether to store the multimedia data currently referred to. This fact not only makes it difficult for the users to grasp the information about the entire data but also requires the terminal devices receiving the data broadcast to be equipped with storage devices with increased capacity.
The above problem that the contents of broadcast multimedia data cannot be known until it is received leads to other problems related to the display attributes of the multimedia data. Such problems include one related to dithering used when displaying JPEG (Joint Photographic Experts Group) images, for example. This problem arises not only in data distribution by broadcast but also in data distribution by communications.
Most mobile receiving terminals, such as portable telephones and mobile terminals, are not provided with sufficient display capabilities because of the cost restriction, etc. For example, receiving terminals capable of displaying only up to 256 colors at one time cannot clearly display some JPEG data images such as photographs with gentle gradation (referred to as JPEG natural painting data hereinafter); the gradation may be displayed like stripes and then the picture will look very clumsy. On the other hand, such receiving terminals can well display JPEG data of line drawings like graphs (referred to as JPEG line drawing data hereinafter) with enhanced contrast.
Techniques for displaying JPEG data as smooth images include dithering. The dithering processing enables JPEG natural painting data to be displayed as an image close in appearance to the original picture. However, when dithering is applied to JPEG line drawing data, the whole becomes unclear (edges become especially unclear) and may be displayed as a deteriorated image with illegible details.
Accordingly, for JPEG data, it is desirable to decide whether or not to apply dithering according to the data type (whether a natural painting or line drawing) before displaying the image.
However, in the above-described multimedia data distribution through broadcasting, the receiving terminal displays the broadcast multimedia data in a mode previously defined in the receiving terminal (with or without dithering). Accordingly the multimedia data may be displayed in an undesirable mode.